Oil quality sensors are known to sense variations in the dielectric constant of petroleum and synthetic lubricating oils. The sensed variations of the dielectric constant of engine oil are compared with a baseline reading of fresh lubricating oil to reveal the presence of contaminants such as soot dissolved in the oil, liquids, emulsified liquids, and particles. The theory behind this sensing technique is that the dielectric constant of the oil is related to the concentration of contaminants in the oil. Assuming the oil is a perfect insulator, the capacitive reactance Xc of the oil can be expressed as:Xc=1/(2πfC), where f is the frequency of a potential applied across the sensor, and C is the capacitance of the oil.
While the capacitive reactance can be measured with little error in non-polar oil, measurement error increases with increasing conductivity of the oil due to a solution current flowing through the oil.
Generally, newly refined oil stock is a non-polar solution. When it is formulated for lubricating oil, various additives are added to improve performance and extend the useful life of the oil. Many of these additives, however, are polar in nature and their polarities increase with increasing temperature. Even as “new” oil reaches operating temperatures, minor solution current can be detected. Solution current also increases as contaminants increase in the oil during use.
Prior art methods and systems utilize an unbalanced alternating current (AC) or static direct current (DC) potential that causes migration of polar contaminants toward oppositely charged sensor electrodes. This contaminant migration results in the build up of contaminants on the electrodes that shield the electrodes from further charge transfer and can contribute to the erroneous measurement of the capacitive reactance of the oil.
A prior approach used to reduce contaminant build up on the electrodes has been to coat one or both of the electrodes with a non-stick surface such as Teflon®. This approach, however, is not effective on polar contaminant migration toward the electrodes. This polar contaminant migration eventually causes liquid dielectric measurement error which can be larger in scale than the inherent or true value of the liquid dielectric measurement. Furthermore, these errors can be exacerbated in low-frequency measurements used in oil quality sensing because the error can manifest itself in an error condition referred to as a double layer capacitance (DLC) error.
Errors resulting from double layer capacitance occur because solid phase conductors only are used as electrodes wherein charges are carried along by a migration of electrons. In solution such as engine lubricating oil, charges are transferred through a migration of ions in the oil. The ions exist in a solution that is in a different phase. Where two phases meet or interface, chemical reactions occur. Such a reaction often includes contaminated molecules such as polarized molecules which disassociate, thereby forming separate positive ions and negative ions within the lubricating oil. The disassociation produces remnants known as solvation shells. Over time, these remnants or artifacts impede ion access and shield the electrodes from further charge transfer. Localized charges, therefore, accumulate around electrodes causing double layer capacitance error.
Another error causing condition associated with liquid dielectric sensing is a condition referred to as surface wetting buildup which results from contaminants accumulated on an insulator surface disposed between two electrodes with the insulator surface parallel to electrostatic lines of force generated by the two electrodes. Surface wetting buildup typically exists on fabricated insulators used to support, seal and separate measurement electrodes at the base of a sensor. In order to reduce the measurement error, frequent cleaning of the insulator surface is needed. This may not be practical in some situations because of the structural complexity of a system. Furthermore, it is desirable to keep a sensor in use with as little contaminant accumulation as possible. Generally, in moderately contaminated oil, contaminants accumulate between electrodes that cause parasitic error capacitance which results in inaccurate measurements from increased sensed capacitance.